The present invention relates to a fuel assembly and a nuclear reactor, especially, to a fuel assembly to be loaded into a boiling water reactor, a lower tie plate of the fuel assembly, a reactor core composed of the fuel assembly, and the boiling water reactor.
In the boiling water reactor, a plurality of fuel assemblies are loaded into a reactor core which is located in the middle of a pressure vessel, and a plurality of cross shaped control rods, of which insertion are regulated by an apparatus of control rod drive, are installed between the fuel assemblies.
Exothermic reaction in a nuclear reactor is maintained by chain reactions of nuclear fission. That is, in the nuclear reactor, a thermal neutron collides with uranium and causes a fission. Kinetic energy of a fission fragment of the uranium is converted mainly to thermal energy. Further, two or three fast neutrons are generated by the fission of uranium. The fast neutron collides with hydrogen atoms in moderator (light water of coolant in the boiling water reactor) several times and is moderated to thermal equilibrium state with atoms composing of the reactor core, and causes next fission of uranium. Subsequently, uranium continues burning by the chain reaction. The reaction is called a nuclear fission. Collision of the uranium with the neutron does not always cause the nuclear fission. Among uranium isotopes existing naturally, the isotope which causes the nuclear fission by collision with the neutron is uranium-235. Isotopic composition ratio of the uranium-235 is about 0.7%, and the rest is uranium-238 which does not cause nuclear fission reaction. Therefore, main commercial nuclear reactors are using enriched uranium, wherein uranium-235 is enriched, as a fuel material.
In a conventional boiling water reactor, a cross-shaped control rod is used to regulate the chain reaction of the nuclear fission by insertion between the fuel assemblies. The control rod contains B.sub.4 C, an absorbing material of neutron. By changing the insertion rate of the control rod into the reactor core, absorption of neutron by the control rod is regulated and the nuclear fission reaction is controlled. Especially, to maintain a scheduled reactivity all through an operation cycle to end of the operation cycle, the fuel material has high reactivity latently at beginning of the operation cycle. In addition to difference of reactivity between at reactor operation and at reactor cold shut down, the high reactivity has possibility to reduce reactor cold shut down margin. Therefore, in view of regulation of excess reactivity at the beginning of the operation cycle and certain holding of the reactor shut down margin, a method to mix a burnable poison such as gadolinia etc. with the fuel material for regulation of reactivity has been adopted in addition to the regulation of reactivity by using control rods.
Further, as another method to regulate the excess reactivity, a method to vary fraction of steam volume in the reactor core (hereinafter called void fraction) is provided. For example, the void fraction in the reactor core is changed by alteration of flow rate of coolant in the reactor core. The change of the void fraction causes variation of moderating effect of the neutron by hydrogen atoms in the moderator and, consequently, variation of reactivity. Accordingly, a method to change void fraction can be utilized for regulation of the reactivity of the reactor core. The void fraction is generally changeable to be large at beginning of the operation cycle and small at end of the cycle. Such change of the void fraction makes harder of neutron spectrum at beginning of the operation cycle than the neutron spectrum at end of the operation cycle, and consequently yield of plutonium as a fissile material is increased. The increasing of yield of plutonium means increment of fissile material in the reactor core, and it makes possible to extend operation period of the nuclear reactor. A method of operation to alter neutron spectrum during the operation cycle so as to extend the operation period is called "spectral shift operation".
Methods of the spectral shift operation based on alteration of ratio of hydrogen to uranium (H/U ratio) during the operation cycle are disclosed in JP-A-57-125390 (1982) (U.S. application Ser. No. 217,275, filing date; Dec. 16, 1980), and JP-A-57-125391 (1982) (U.S. application Ser. No. 217,061, filing date; Dec. 16, 1980).
Publications described above are indicating that the spectral shift operation is performed with using a water displacer rod which is installed separately in addition to a control rod. That is, the water displacer rod is inserted into a fuel assembly to decrease the H/U ratio at beginning of an operation cycle, and the water displacer rod is withdrawn from the fuel assembly to increase the H/U ratio at end of the operation cycle. At the beginning of the operation cycle when the H/U ratio is small, yield of fissile plutonium is increased as same as the case of the large void fraction.
The prior art described above necessitates a driving apparatus to handle the water displacer rod separately in addition to a control rod driving apparatus to handle the control rod, and consequently structure of the nuclear reactor becomes complex. A plurality of water displacer rods are installed instead of only one rod.
A fuel assembly with which the spectral shift operation is possible by altering void fraction in the fuel assembly is disclosed in EP-A-0205162. The fuel assembly has an orifice having a plurality of round rods which are installed in a way to cross a coolant path at inside of a lower tie plate.
JP-A-57-125390 (1982) and JP-A-57-125391 (1982) indicate a spectral shift operation of a pressurized water reactor, while EP-A-0205162 indicates a spectral shift operation of a boiling water reactor. EP-A-0205162 describes a structure installing the orifice at a coolant path inside of fuel support fittings which support the fuel assembly as another example.
JP-A-60-177293 (1985) discloses a structure to perform the spectral shift operation by altering void fraction in a gap (hereinafter called water gap region) which is formed between each of loaded fuel assemblies in a reactor core of a boiling water reactor, concretely saying, water gap region which is formed between channel boxes of each fuel assembly. Coolant flows in the water gap region as well as in the fuel assembly. The coolant is leaked into the water gap region from a gap between a lower tie plate of the fuel assembly and the channel box, between the lower tie plate and a fuel support fitting, and between the fuel support fitting and a reactor core support plate, and so on. In JP-A-60-177293 (1985), a through hole connecting to the water gap region is provided at the reactor core support plate, and further a flow regulating valve is installed at the reactor core support plate to alter the void fraction in the water gap region. The flow regulating valve regulates the flow rate of the coolant from the through hole to the water gap region. That is, the valve disc of the flow regulating valve closes the through hole by action of a spring which is attached to the valve. The condition described above is caused at a case when flow rate of the coolant to the fuel assembly is small. When the flow rate of the coolant is increased, the valve disc moves upward to open the hole and coolant is provided to the water gap region through the hole. At beginning of an operation cycle, the through hole is closed by the valve disc and void fraction in the water gap region is increased. At end of the operation cycle, the through hole is opened and the void fraction in the water gap region is decreased. Effect of the spectral shift operation is larger when void fraction outside of the fuel assembly, namely void fraction in the water gap region, is altered than a case when void fraction in side of the fuel assembly is altered.
The method disclosed in JP-A-60-177293 uses a simpler structure than other prior art which uses water displacer rod. However, the flow regulating valve which is disclosed in JP-A-60-177293 uses recovering power of a spring and causes a problem that the spring loses its recovering power as a result of neutron irradiation.